Contact lenses made from silicone materials have been investigated for a number of years. Such materials can generally be subdivided into two major classes, namely hydrogels and non-hydrogels. Non-hydrogels do not absorb appreciable amounts of water; whereas, hydrogels can absorb and retain water in an equilibrium state. Hydrogels generally have a water content greater than about five weight percent and more commonly between about ten to about eighty weight percent. Hydrogel silicone contact lenses tend to have relatively hydrophobic, non-wettable surfaces. Thus, those skilled in the art have long recognized the need for rendering the surface of silicone contact lenses hydrophilic to improve their biocompatibility or wettability by tear fluid in the eye. This in turn is necessary to improve the wear comfort of contact lenses. In the case of continuous-wear lenses, lenses worn both day and night, the comfort of the lens and thus its surface is especially important. Furthermore, the surface of a continuous-wear lens must be designed not only for comfort, but to avoid corneal edema, inflammation and other adverse effects that may be caused by continuous wear of lenses made of unsuitable materials.
Silicone lenses have been subjected to plasma surface treatment to improve their surface properties, e.g., surfaces have been rendered more hydrophilic, deposit resistant, scratch resistant, and the like. Examples of common plasma surface treatments include subjecting the contact-lens surfaces to plasma of: an inert gas or oxygen (see, for example, U.S. Pat. Nos. 4,055,378; 4,122942; and 4,214,014); various hydrocarbon monomers (see, for example, U.S. Pat. No. 4,143,949); and combinations of oxidizing agents and hydrocarbons, e.g., water and ethanol (see, for example, WO 95/04609 and U.S. Pat. No 4,632,844). Sequential plasma surface treatments are also known such as those comprising a first treatment with a plasma of an inert gas or oxygen, followed by a hydrocarbon plasma (see, for example, U.S. Pat. Nos. 4,312,575 and 5,326,584).
U.S. Pat. No. 4,312,575 to Peyman et al. discloses a process for providing a barrier coating on a silicone or polyurethane lens by subjecting the lens to an electrical glow discharge (plasma) process in the presence of a hydrocarbon atmosphere followed by subjecting the lens to oxygen during flow discharge, thereby increasing the hydrophilicity of the lens surface. Peyman et al. teach that the process should be carried out in the absence of oxygen and that alkanes such as methane or butane are preferred. In contrast, U.S. Pat. No. 4,632,844 to Yanagihara teaches plasma treatment of a contact lens with various hydocarbons in the presence of oxygen. Yanagihara includes the use of butadiene (Example 14 and claim 3), but does not specify application to silicone materials in the examples. WO 94/29756 mentions an oxygen/methane/oxygen multi-step process for surface treating an RGP (Rigid Gas Permeable) silicone lens, but does not provide any parameters.
Although such surface treatments have been disclosed for modifying the surface properties of silicone contact lenses, the results have been inconsistent and problematic, in some cases preventing commercialization of high Dk silicone hydrogel lens materials. The coating of such lenses is complicated by the fact that, although silicone hydrogel lenses may be plasma-treated in an unhydrated state, hydrogels subsequently swell when hydrated, unlike their non-hydrogel counterparts. In fact, hydration commonly may cause the lens to swell about ten to about twenty percent in volume, depending upon the water content of the lens. Such swelling of the lens and subsequent autoclaving, a common form of sterilizing lenses, can adversely affect and even remove the desired coating with loss of properties, a phenomenon referred to as delamination.
Thus, it is desired to provide a silicone hydrogel contact lens with an optically clear, hydrophilic coating upon its surface which will withstand subsequent hydration and autoclaving. In the case of a silicone hydrogel lens that is highly permeable to oxygen for continuous wear, it would be highly desirable to form a coating that is water wettable and durable, such that the lens is safe and comfortable to wear, allowing for continuous (night and day) wear of the lens for a week or more without adverse effects to the cornea.